1. Technical Field
The present invention relates to a positioning method, a program of the positioning method, and a positioning device.
2. Related Art
As a positioning system using a positioning signal, a global positioning system (GPS) is widely known, and is used as a positioning device incorporated in, for example, a portable phone or a vehicle navigation system. In GPS, the positioning calculation is performed for obtaining values of four parameters, namely three-dimensional coordinate values representing the location of the device and a clock error, based on the information such as the locations of a plurality of GPS satellites or pseudo-ranges between the GPS satellites and the device, thereby positioning the present location of the device.
In the positioning system described above, in order for extracting and capturing the positioning signal from a received signal, there is generally used a method of executing correlation calculation between the received signal which is spread-modulated with a type of spread codes known as a PRN code, and a replica code of the PRN code, and then detecting a code phase with a maximum correlation value. However, if the code phase fails to be correctly detected, it is not possible to correctly obtain the pseudorange used in the positioning calculation, which causes degradation of positioning accuracy.
Therefore, for example, JP-A-2000-312163 (Document 1) discloses a technology in which the correlation calculation between the received signal and each of a plurality of replica codes of the PRN codes, the phase of which changes every chip, (hereinafter referred to as the “first correlation calculation”) is executed to obtain the phase search range expected to maximize the correlation value, and further, the correlation calculation between the received signal and each of a plurality of replica codes, the phase of which changes every 0.1 chip, is executed, thereby detecting the code phase.
In the technology disclosed in the Document 1, the correlation calculation is performed with different resolutions, thereby achieving improvement of detection accuracy of the code phase. However, in such an environment as an indoor environment in which the positioning signal thus received is in a weak electric field, it is difficult to detect a correlation maximum value in a weak signal, and in some cases, a correlation value once determined to be the maximum value in the first correlation calculation is not a true maximum value.
In this case, since the second correlation calculation is executed with a higher resolution in the phase search range thus set based on the correlation value, which is not the true maximum value (wrong phase search range), in the technology described above, there arises a problem that it is not possible to correctly detect the code phase, resulting in degradation of the positioning accuracy.